I would go to nvidia.com and check i think that these are the best cards, can be little high on cost but well worth it. just make sure the motherboard can handle the card.
Dell Optiplex GX260 (Small Mini-Tower Computer)
I know i have a agp video slot, but not sure if it is a 4x or 8x? And what does that mean? And i need it to support directX (or something like that) and 3d effects?